Process using carbonate precipitation



PROCESS USING CARBONATE PRECIPITATION Harrison S. Brown and Orville F. Hill, Oak Ridge, Tenn.,

and Arthur H. Jatitey, Chicago, 111., assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Application July 30, 1945, Serial No. 607,920

9 Claims. (Cl. 2314.5)

The present invention relates to a process for the separation of a transuranic element from impurities commonly associated therewith. More particularly it is concerned with the separation of plutonium from uranium, fission products and other impurities normally present in solutions of neutron irradiated uranium.

It is known that plutonium can be produced in small quantities by the bombardment of natural uranium with neutrons. The designation plutonium or element 94 as used throughout the present description refers to the transuranic element having an atomic number of 94. The expression 94 means the isotope of element 94 that has an atomic weight or mass of 239. Similarly, the terms element 93 or Np refer to the new element known as neptunium having an atomic number of 93.

Uranium is composed of three isotopes, namely, U U and U the latter being present in excess of 99 percent of the whole. When U is subjected to the action of slow or thermal neutrons, a fourth isotope, U is produced which has a half-life of 23 minutes and undergoes beta decay to Np which decays further by beta radiation with a half-life of 2.3 days to yield plutonium. In addition to the formation'of 94 there are simultaneously produced other elements of lower atomic weight known as fission fragments. These fission fragments are composed of two distinct element groups, i. e., alight and a heavy element group. The light group contains elements having atomic numbers of between about 35 and 46 while the heavier group is composed of elements of atomic numbers varying between about 51 and 60. The elements of these groups as originally produced are considerably overmassed and undercharged, and hence are highly unstable. By beta radiation, however, they quickly transform themselves into isotopes of these various elements having longer half-lives. The resulting materials are commonly known as fission products.

The various radioactive fission products have halflives ranging from a fraction of a second to thousands of years. Those having very short half-lives may be substantially eliminated by aging the material for a reasonable period before handling. Those with very long halflives do not have sufficiently intense radiation to endanger personnel protected by moderate shielding. On the other hand, the fission products having half-lives ranging from a few days to a few years have dangerously intense radia tions which cannot be eliminated by aging for practical storage periods. These products are chiefly radioactive isotopes of Sr, Y, Zr, Cb, and Ru of the light group and Te, I, Cs, Ba, La, Ce, and Pr of the heavy group.

It may be readily seen that plutonium. produced as generally set fourth above. is contaminated with considerable quantities of uranium an-d'fission products. In

fact, the plutonium constitutes only a very minor portion.

of the irradiated'masn'i. e., less than one per centthereof. In view of such a low concentration of plutonium in the irradiated metal, it becomes apparent that the procedure employed to recover thatelement must be highly efiicient in order to beat all practicable.

nited States Patent Patented Oct. 30, 1956 There have been devised a number of procedures for the removal and concentration of plutonium from extremely dilute solutions thereof. In general, such methods involve the formation of various insoluble compounds in said dilute solutions capable of carrying plutonium in the reduced state. The carrier precipitate and plutonium thus obtained are then dissolved and the plutonium is oxidized to PuOz in which state of oxidation it is soluble in the presence of said carrier. Under these conditions, the plutonium remains in solution and the fission products are removed when the carrier is added. Thereafter, the dissolved plutonium is reduced to a valence state in which it is carriable by the aforesaid carrier and removed from solution in the form of a carrier precipitate which may again be dissolved and the plutonium purified further if considered necessary or desirable by repeating the above cycle.

It is an object of this invention to provide a convenient and efiicient method of recovering plutonium from impurities commonly associated therewith.

A further object of the present invention is to efiect recovery of element 94 from uranium-containing solutions by means of a process based on the diflerence in solubility of the carbonates of these two elements at a comparatively low pH.

Broadly, the present invention involves first precipitating uranium from a solution containing plutonium and uranium at a pH of between about 2.5 and 4.0 in the presence of carbonate ions. The resulting precipitate which probably consists essentially of basic uranous carbonate is then removed from solution by centrifugation, filtraton or other convenient means and the mother liquor adjusted to a pH of between about 5.5 and 7.5 by the additionof a suitable carbonate or bicarbonate. Thereafter the solution is contacted with a suitable carrier for element 94 and the latter thus removed therefrom in accordance with the procedure generally set forth above. The carrier precipitate obtained in this manner is dissolved and the plutonium present therein is oxidized to the hexavalent state, after which it may be readily separated from the carrier by converting the latter to an insoluble compound.

In its preferred embodiment, the process of the present invention consists essentially of first dissolving neutron irradiated uranium in hydrochloric acid forming a solution in which the uranium is present in a concentration of about 5 percent. While at least the majority of the plutonium present in the resulting solution is in the trivalent state under such circumstances, it is generally desirable to introduce a suitable reducing agent such as sodium nitrite, hydroxylarnine, uranous ion, an aqueous sulphur dioxide solution, or the like, in order to insure the complete absence of urany-l ions which, if present, will not be removed at the pH levels employed in the presence of carbonate ions. The solution containing trivalent plutonium is next adjusted to a pH of between about 2.7 and 3.0 by the addition of solid sodium bicarbonate, or a similar solid bicarbonate, whereupon a acid aud t-he plutonium contained therein oxidized, by

the addition of sodium dichromate, to the hexavalent state. The resulting solution is then adjusted to a pH of between about 6.5 and 7.2 with solid sodium carbonate and the fission products present are removed by forming a lanthanum carbonate precipitate therein. The latter is separated from the mother liquor by filtration and, if desired, the plutonium contained in such solution may be further separated from any fission products which might be present by repeating the above cycle until a plutonium solution of the desired purity is reached.

It will be apparent that the procedure set forth above has considerable advantage inasmuch as all pr cipitates involved can be readily dissolved and reprecipitated.

The process of the present invention may be further illustrated by the following specific example.

Example Neutron-irradiated uranium was dissolved in a 40 percent hydrochloric acid to produce a percent uranous chloride solution after which sufficient solid sodium bicarbonate was introduced to lower the acidity thereof to a pH of 2.7 whereupon the uranium present precipitated in the form of its basic carbonate. In this step approximately 66 percent of the gamma activity (fission product activity) was removed from the solution. The precipitate thus obtained was separated from the supernatant liquid by filtration and the residue washed with a solution of sodium bicarbonate after which the washings were combined with the filtrate and the pH of the resulting solution was increased to 6.5 by the introduction of additional solid sodium bicarbonate. The plutonium present in the solution was next removed by adding thereto a solution of lanthanum nitrate containing La+ ions in a concentration of about 1 mg. per ml. T he lanthanum nitrate solution was added until precipitation of lanthanum carbonate appeared to be complete after which the plutonium-containing precipitate thus obtained was separated from the mother liquor by filtration and then dissolved in a 40 percent hydrochloric acid solution. To the latter was added an aqueous solution of sodium dichromate in an amount sufficient to yield a final mixture which was approximately 2.4 N with respect to hydrochloric acid and 0.1 M with respect to sodium dichromate. The solution was next digested for about one half hour at 65 C. to effect oxidation of the Pitto Pu+ Thereafter the pH of the solution was adjusted to a value of 6.5 by the addition of solid sodium bicarbonate. The fission products were then removed by forming a lanthanum carbonate carrier in situ as previously described and the resulting precipitate was filtered otf. Sufficient sulphur dioxide in the form of a 10 percent aqueous solution was next added to reduce the l exavalcnt plutonium to the trivalent state in which form it is carriable. The pH was again adjusted to 6.5 by the addition of sodium bicarbonate and then the trivalent plutonium was separated from solution by means of 1a lanthanum carbonate carrier in accordance with the procedure set forth above. The precipitate thus obtained was found to contain 94 percent of the plutonium present in the original hydrochloric acid solution.

It will be apparent to those skilled in the art that various modifications of the present invention exist. For example, if desired, the trivalent plutonium can be oxidized to the hexavalent state prior to the removal of fission products and then reduced to the carriable state as shown by the foregoing description. In general it may he said that any process for the separation of plutonium from uranium and other impurities normally associated therewith based upon the difference in solubility of the carbonates of plutonium and uranium at given pH ranges is to be considered as lying within the scope of the present invention.

What is claimed is:

l. in a process for the recovery of plutonium values from neutron irradiated uranium, the steps which comprise dissolving said uranium in hydrochloric acid, adding carbonate ions thereby adjusting the pH of the resulting solution to a value of between about 2.5 and 4.0, removing the precipitate thus formed, contacting the resulting solution with a source of La ions, thereafter adding carbonate ions to adjust the pH of the solution to a value of between about 5.5 and 7.5 and removing the plutonium values from solution by means of the lanthanum carbonate carrier thus formed.

2. In a process for the recovery of fission product values from neutron irradiated uranium containing plutonium, the steps which comprise dissolving said uranium in hydrochloric acid, adding a water-soluble bicarbonate thereby adjusting the pH of the resulting solution to a value of between about 2.5 and 4.0, removing the precipitate thus formed, contacting the resulting solution with a source of La ions, thereafter adding watersoluble bicarbonate to adjust the pH of the solution to a value of between about 5.5 and 7.5, removing the fission product values and plutonium values from solution by means of the lanthanum carbonate carrier thus formed, dissolving said precipitate in hydrochloric acid, adding sodium dichromate to said solution whereby all plutonium present is converted to the hexavalent state, adding sodium carbonate to adjust the pH value of the solution to about 6.5, adding a soluble lanthanum salt whereby lanthanum carbonate and fission product carbonates are precipitated while the plutonium values remain in solution.

3. In a process for the recovery of plutonium values from acid solutions containing uranium values and plutonium values, the steps which comprise adding a water-soluble bicarbonate thereby adjusting the pH of the solution to a value of between about 2.5 and 4.0, removing the precipitate thus formed, contacting the resulting solution with a source of La+ ions, thereafter adding water-soluble bicarbonate to adjust the pH of the solution to a value of between about 5.5 and 7.5, and removing the plutonium values from solution by means of the lanthanum carbonate carrier thus formed.

4. In a process for the recovery of plutonium values from hydrochloric acid solutions containing uranium values and plutonium values, the steps which comprise adding a water-soluble bicarbonate thereby adjusting the pH of the solution to a value of between about 2.5 and 4.0, removing the precipitate thus formed, contacting the resulting solution with a source of La+ ions, thereafter adding water-soluble bicarbonate thereby adjusting the pH of the solution to a value of between about 5.5 and 7.5, and removing the plutonium values from solution by means of the lanthanum carbonate carrier thus formed.

5. In a process for the recovery of plutonium values from hydrochloric acid solutions containing plutonium values and tetravalent uranium values, the steps which comprise adding a water-soluble bicarbonate thereby adjusting the pH of the solution to a value of between 2.7 and 3.0, removing the precipitate thus formed, contacting the resulting solution with a source of La+ ions, thereafter adding water-soluble bicarbonate thereby adjusting the pH of the solution to a value of between about 6.5 and 7.2, and removing the plutonium values from solution by means of the lanthanum carbonate carrier thus formed.

6. In a process for the recovery of plutonium values from hydrochloric acid solutions containing plutonium values and tetravalent uranium values, the steps which comprise adding sodium bicarbonate thereby adjusting the pH of the solution to a value of between about 2.5 and 4.0, removing the precipitate thus formed, contacting the resulting solution with a source of La+ ions, thereafter adding sodium bicarbonate to adjust the pH of the solution to a value of between 5.5 and 7.5, and removing the plutonium values from solution by means of the lanthanum carbonate carrier thus formed.

7. In a process for the recovery of plutonium values from solutions containing plutonium values, fission product values, and tetrava-lent uranium values, the steps which comprise adding a water-soluble bicarbonate thereby adjusting the pH of said solution to a value of between about 2.5 and 4.0, removing the precipitate thus formed, contacting the resulting solution with a source of La+ ions, thereafter adding water-soluble bicarbonate thereby increasing the pH of said solution to a value of between about 5.5 and 7.5, removing the lanthanum carbonate precipitate which contains plutonium and fission product values, dissolving the precipitate in aqueous acid and oxidizing the plutonium to the hexavalent state, thereafter adding carbonate ions thereby adjusting the pH of the solution to a value of between about 6.5 and 7.2, contacting this solution with a source of La ions, and removing said fission product values by carrying the latter on the lanthanum carbonate precipitate thus formed.

8. In a process for the recovery of plutonium values from hydrochloric acid solutions containing uranium uct values and tetravalent uranium values, the steps which comprise adding a water-soluble bicarbonate thereby adjusting the pH of the solution to a value of between about 2.5 and 4.0, removing the precipitate thus formed, contacting the resulting solution with lanthanum nitrate, thereafter adding water-soluble bicarbonate to adjust the pH of the solution to a value of between about 5.5 and 7.5, and removing the plutonium values from solution by means of the lanthanum carbonate carrier thus formed.

9. In a process for the recovery of plutonium values from solutions containing plutonium values, fission product values and tetravalent uranium values, the steps which comprise adding water-soluble bicarbonate thereby adjusting the pH of said solution to a value of between about 2.5 and 4.0, removing the precipitate thus formed, contacting the resulting solution with lanthanum nitrate, thereafter adding water-soluble bicarbonate thereby increasing the pH of said solution to a value of between about 5.5 and 7.5, removing the lanthanum carbonate precipitate which contains plutonium and fission product values, dissolving the precipitate in aqueous acid and oxidizing the plutonium to the hexavalent state, thereafter adding carbonate ions to adjust the pH of the solution to a value of between about 6.5 and 7.2, contacting the resulting solution with lanthanum nitrate and removing said fission product values by carrying the latter on the lanthanum carbonate precipitate thus formed.

No references cited. 

1. IN A PROCESS FOR THE RECOVERY OF PLUTONIUM VALUES FROM NEUTRON IRRADIATED URANIUM, THE STEPS WHICH COMPRISE DISSOLVING SAID URANIUM IN HYDROCHLORIC ACID, ADDING CARBONATE IONS THEREBY ADJUSTING THE PH OF THE RESULTING SOLUTION TO A VALUE OF BETWEEN ABOUT 2.5 AND 4.0, REMOVING THE PRECIPITATE THUS FORMED, CONTACTING THE RESULTING SOLUTION WITH A SOURCE OF LA+3 IONS, THEREAFTER ADDING CARBONATE IONS TO ADJUST THE PH OF THE SOLUTION TO A VALUE OF BETWEEN ABOUT 5.5 AND 7.5 AND REMOVING THE PLUTONIUM VALUES FROM SOLUTION BY MEANS OF THE LANTHANUM CARBONATE CARRIER THUS FORMED. 